1. Field of the Invention
The present invention relates to a camera calibration method and a camera calibration apparatus adapted for the camera calibration method.
2. Description of the Related Art
In a stereoscopic vision system, the position of an object present in a real space is comprehensible on the basis of data of an image photographed by a camera. Therefore, in order to determine the position of an object present in a three dimensional real space by a stereoscopic vision system on the basis of an image photographed by a camera, it is necessary to calibrate the camera (camera calibration) so as to obtain an accurate relation between the image and directions thereof in the real space. However, due to the image distortions or the phenomenon such as incident rays do not intersect at one point or the like caused by a lens, it is necessary to perform the camera calibration with the non-pinhole property taken into consideration. Such camera calibration device, for example, has been disclosed in Japanese Patent Laid-open No. 2004-48399 (referred to as Patent Document 1 hereinafter).
The camera calibration device is provided with a rotation stage for varying the position of a point light source disposed in front of the camera on XYZ axes and a detection unit for detecting the position of the point light source on each axis. The rotation operation for rotating the rotation stage so that the image of the point light source falls on the center of a pixel is performed to move the point light source far away from and close to the camera for two times to deduce the trajectory of an incident ray from the point light source, thereby, to perform the camera calibration and set internal parameters of the camera.
However, according to the camera calibration method disclosed in Patent Document 1, it is necessary to deduce the trajectory of an incident ray for each individual pixel; therefore, a long time will be needed to calibrate the camera. Specifically, for an individual pixel, the operation for moving the image of the point light source to the center of the pixel will cost about 1 second; therefore, to calibrate a camera with 640×480 pixels will cost about 7 days. Since there are multiple cameras in the stereoscopic vision system, the calibration time will be as long as the time for calibrating one camera multiplied by the number of cameras in the stereoscopic vision system.
To perform the camera calibration with such a long time is practically impossible. For a common camera, linear interpolation is possible for a number of pixels; therefore, the camera calibration is performed every several pixels, and the linear interpolation is performed on the several pixels. Nevertheless, to calibrate one camera with 640×480 pixels for every 5 pixels each time will cost about 7 days. Also, the calibration accuracy will be decreased if the number of pixels is made greater; therefore, shortening the calibration time by making the number of pixels greater than that is practically impossible. Thereby, the camera calibration method which costs such a long time cannot cope with the mass production of cameras.
Moreover, according to the camera calibration method disclosed in Patent Document 1, the trajectory of the incident ray is deduced for each individual pixel by the usage of a convergence processing based on feedback loop; therefore, if the pixel corresponding to the direction of incident ray is of plural numbers, they may not converge. For example, when a camera has an extremely distorted shield or a plurality of lenses, for the same incident ray, the pixel corresponding to the same incident ray is of plural numbers.